Calibration and Localization of Optically Pumped Magnetometers Using Electromagnetic Coils

Abstract

In this paper, we propose a method to estimate the position, orientation, and gain of a magnetic field sensor using a set of (large) electromagnetic coils. We apply the method for calibrating an array of optically pumped magnetometers (OPMs) for magnetoencephalography (MEG). We first measure the magnetic fields of the coils at multiple known positions using a well-calibrated triaxial magnetometer, and model these discreetly sampled fields using vector spherical harmonics (VSH) functions. We then localize and calibrate an OPM by minimizing the sum of squared errors between the model signals and the OPM responses to the coil fields. We show that by using homogeneous and first-order gradient fields, the OPM sensor parameters (gain, position, and orientation) can be obtained from a set of linear equations with pseudo-inverses of two matrices. The currents that should be applied to the coils for approximating these low-order field components can be determined based on the VSH models. Computationally simple initial estimates of the OPM sensor parameters follow. As a first test of the method, we placed a fluxgate magnetometer at multiple positions and estimated the RMS position, orientation, and gain errors of the method to be 1.0 mm, 0.2°, and 0.8%, respectively. Lastly, we calibrated a 48-channel OPM array. The accuracy of the OPM calibration was tested by using the OPM array to localize magnetic dipoles in a phantom, which resulted in an average dipole position error of 3.3 mm. The results demonstrate the feasibility of using electromagnetic coils to calibrate and localize OPMs for MEG.

Magnetoencephalography with optically pumped magnetometers (OPM-MEG): the next generation of functional neuroimaging

“…This system can also work dynamically, with constant feedback allowing drifts in field over time to be accounted for. In addition, recent work has shown that this high fidelity field control can also be exploited for calibration, as well as to identify the location and orientation of the sensors themselves [ 68 ].…”

An Iterative Implementation of the Signal Space Separation Method for Magnetoencephalography Systems with Low Channel Counts

“…In practice, an SSS-guided calibration is often used to ensure optimal performance, but the operational principle of OPMs, and the potential for many different array configurations means SSS-guided approaches may be challenging to implement (as sensor calibration may vary depending on parameters, including the density of atoms in the OPM vapour cell and the background field experienced by the sensor). Methods based on the use of electromagnetic coils have been proposed in mitigation [ 24 ]. All these issues will need to be addressed to fully realise the potential of OPM-MEG.…”

Improved non-invasive detection of ictal and interictal epileptiform activity using Optically Pumped Magnetometers

“…A compromise may be the use of a limited number of different-sized rigid helmets, with holders in which OPMs can be pressed onto the scalp (Zetter et al, 2019). The depth of the OPMs within their holder could be measured manually, or sensor positions and orientations could be determined automatically, using either an optical approach (Gu et al, 2021) or utilising the field-nulling coils (Iivanainen et al, 2022). The latter approach has the advantage that any changes in the orientations of the OPM’s sensitive axes due to crosstalk from neighbouring OPMs are already taken into account.…”